I'm an old time user of freemind / freeplane / docear. I would say I have limited knowledge about mindmapping.Suppose you have selected a couple of lines of text on a webpage, and you are executing a drag and drop onto a mindmap node.I seem to remember there was a possibility to choose for the text to either be split (per line to separate nodes) or to be kept together in a single node.

If you have an editor opened the text will be inserted as a block but if you simply drop it onto a node the text will be split. I checked that with Freeplane 1.3 but I think that this functionality has not changed.

under Windows I dragged multiple lines from the browser into the inline editor and it worked as described, but I did not try the SimplyHtml editor. Under Linux both works with most recent 1.3.x version. If you want to loose all formatting on copying I think you have to use an text editor as an intermediate drop location, but I may be wrong on that.